Stockholders' Equity (Additional Information Regarding Options Outstanding) (Details) (USD $)	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2010
Options outstanding, Number outstanding	1,814,599	2,632,115	2,152,183	1,663,497
Options outstanding, Average remaining contractual life (years)	6 years 11 months 1 day	7 years 2 months 27 days	6 years 10 months 28 days	7 months 14 days
Options outstanding, Weighted-average exercise price	$ 2.83	$ 3.18	$ 3.38	$ 3.34
Options exercisable, number outstanding	1,367,834	1,627,079
Options exercisable, weighted-average exercise price	$ 3.15	$ 3.49
$0.86 - $1.26 [Member]
Range of exercise price, lower limit	$ 0.86
Range of exercise price, upper limit	$ 1.26
Options outstanding, Number outstanding	235,000
Options outstanding, Average remaining contractual life (years)	8 years 9 months 18 days
Options outstanding, Weighted-average exercise price	$ 1.16
Options exercisable, number outstanding	15,000
Options exercisable, weighted-average exercise price	$ 1.10
$1.34 - $1.80 [Member]
Range of exercise price, lower limit	$ 1.34
Range of exercise price, upper limit	$ 1.80
Options outstanding, Number outstanding	181,500
Options outstanding, Average remaining contractual life (years)	8 years 9 months 11 days
Options outstanding, Weighted-average exercise price	$ 1.42
Options exercisable, number outstanding	132,352
Options exercisable, weighted-average exercise price	$ 1.42
$1.85 - $2.15 [Member]
Range of exercise price, lower limit	$ 1.85
Range of exercise price, upper limit	$ 2.15
Options outstanding, Number outstanding	183,194
Options outstanding, Average remaining contractual life (years)	7 years 7 months 17 days
Options outstanding, Weighted-average exercise price	$ 1.94
Options exercisable, number outstanding	129,943
Options exercisable, weighted-average exercise price	$ 1.94
$2.25 - $2.55 [Member]
Range of exercise price, lower limit	$ 2.25
Range of exercise price, upper limit	$ 2.55
Options outstanding, Number outstanding	122,000
Options outstanding, Average remaining contractual life (years)	4 years 15 days
Options outstanding, Weighted-average exercise price	$ 2.51
Options exercisable, number outstanding	122,000
Options exercisable, weighted-average exercise price	$ 2.51
$2.60 - $2.60 [Member]
Range of exercise price, lower limit	$ 2.60
Range of exercise price, upper limit	$ 2.60
Options outstanding, Number outstanding	288,707
Options outstanding, Average remaining contractual life (years)	6 years 4 days
Options outstanding, Weighted-average exercise price	$ 2.60
Options exercisable, number outstanding	283,344
Options exercisable, weighted-average exercise price	$ 2.60
$2.65 - $2.86 [Member]
Range of exercise price, lower limit	$ 2.65
Range of exercise price, upper limit	$ 2.86
Options outstanding, Number outstanding	9,625
Options outstanding, Average remaining contractual life (years)	6 years 5 months 27 days
Options outstanding, Weighted-average exercise price	$ 2.69
Options exercisable, number outstanding	8,541
Options exercisable, weighted-average exercise price	$ 2.69
$2.90 - $2.90 [Member]
Range of exercise price, lower limit	$ 2.90
Range of exercise price, upper limit	$ 2.90
Options outstanding, Number outstanding	217,500
Options outstanding, Average remaining contractual life (years)	8 years 4 days
Options outstanding, Weighted-average exercise price	$ 2.90
Options exercisable, number outstanding	159,128
Options exercisable, weighted-average exercise price	$ 2.90
$3.00 - $3.46 [Member]
Range of exercise price, lower limit	$ 3.00
Range of exercise price, upper limit	$ 3.46
Options outstanding, Number outstanding	90,962
Options outstanding, Average remaining contractual life (years)	6 years 11 months 5 days
Options outstanding, Weighted-average exercise price	$ 3.13
Options exercisable, number outstanding	71,263
Options exercisable, weighted-average exercise price	$ 3.15
$3.56 - $3.56 [Member]
Range of exercise price, lower limit	$ 3.56
Range of exercise price, upper limit	$ 3.56
Options outstanding, Number outstanding	190,762
Options outstanding, Average remaining contractual life (years)	7 years 4 days
Options outstanding, Weighted-average exercise price	$ 3.56
Options exercisable, number outstanding	153,186
Options exercisable, weighted-average exercise price	$ 3.56
$3.75 - $6.70 [Member]
Range of exercise price, lower limit	$ 3.75
Range of exercise price, upper limit	$ 6.70
Options outstanding, Number outstanding	295,349
Options outstanding, Average remaining contractual life (years)	5 years 1 month 2 days
Options outstanding, Weighted-average exercise price	$ 5.29
Options exercisable, number outstanding	293,077
Options exercisable, weighted-average exercise price	$ 5.31